Adaptive transceiver system

ABSTRACT

A method for forming signals at a transceiver having at least two transmit and receive chains, the method comprising the steps of: (a) determining the phase difference and relative amplitude of signals from a set comprising a plurality of mobile stations as received through the receive chains, (b) receiving from each of at least one of the mobile stations messages indicative of the strength or quality of signals as received by the respective mobile station from the transceiver and on the basis of those messages determining a phase offset and amplitude distortion, internal to the transceiver, resulting from the differences in the instrumental properties of the receiver and transmitter chains in the transceiver; and (c) transmitting signals from each of the transmitter chains by applying to each transmitter chain amplitude weights and signal delays, selected on the basis of the determined phase offset and amplitude distortion, and received relative amplitudes and phase differences.

This invention relates to an adaptive transceiver system. The system issuitably capable of determining transmit weights for multipletransmission chains in accordance with characteristics of receivedsignals, suitably in order to calibrate a unit such as a base station.

In FIG. 1 the principle behind a beamforming antenna system isillustrated. Transmitter 1 is a conventional transmitter. It transmits aradio signal from an antenna 2. The general pattern of the transmittedsignal is a lobe shown at 3. Typically the width α of the transmittedbeam covers the whole, typically 120′ wide, sector. Transmitter 4 is abeamforming transmitter. It includes two antennas 5, 6 each of whichtransmits signals over a similar lobe 7, 8 covering the whole sector. Inthe beamforming transmitter the same signal is transmitted from eachantenna 5, 6, but the relative phase of the signals is selected so thatthe signals interfere constructively over a relatively narrow beam 9. Bycontrolling the relative phase and amplitudes of the signals the beam ofconstructive interference can be directed towards a desired receiver 10.It is emphasized that beamforming provides just an example of adaptivetransceiver systems.

In situations where transmitted signals are intended for a singlereceiver, adaptive systems such as beamforming have significantpotential advantages over conventional transmitter systems. Since agreater proportion of the transmitted energy is offered to the receiver,an adaptive system demands less total transmitted power, and causes lessinterference to other receivers. Furthermore, proposals for the 3G/WCDMA(third generation/wide-band code division multiple access) mobilecommunication system makes use of multiple antennas to provide anadditional diversity.

One situation in which adaptive antennas could be particularly useful ismobile phone systems. Mobile phone basestations transmit signals thatare directed to individual mobile terminals. Reducing transmitted powerand interference is especially desirable in mobile phone systems becausea reduction in expected interference would mean higher network capacity.However, a major difficulty in the implementation of adaptivetransmitters at the basestations of mobile phone systems is thecalculation of the relative phase and amplitudes of the signals thatmust be transmitted from the antennas so as to adapt the transmission toa desired mobile station.

As an example of adaptive antennas, in FIG. 2 a beamforming basestationfor a mobile phone system is considered. The basestation includes a pairof antennas 20, 21. Each antenna is connected via a duplexer 22, 23 to atransmit chain 24, 25 and a receive chain 26, 27. The receive chainsinclude a low noise amplifier 28, 29 and a mixer 30, 31 fordownconverting the received signal to baseband. In practice two or moredownconversion and amplification stages may be employed. The basebandsignals are converted to digital form by A-to-D converters 32, 33 forfurther processing. In the transmission sections a signal fortransmission is generated in digital form at 34. The signal is split tothe two antennas and converted to analogue by D-to-A converters 35, 36.In the transmit chains the analogue signals for transmission areupconverted by mixers 37,38 and amplified by amplifiers 39, 40 beforebeing applied to the respective antenna via the duplexers 22, 23. It isnoted that in order to keep this example simple only phase control hasbeen considered in FIG. 2. This do not, however, mean that amplitudecontrol would not be applicable. A phase control unit 41 determines thephase offset required to direct a beam to a desired mobile station. Thephase control unit forms a phase control signal 42 which is applied tocontrol a phase control unit 43 located in one branch of the digitalinput. The delay unit inserts a phase offset to antenna 21 so as tocause a phase offset between the signals transmitted from the antennas.

To direct the beam towards a desired mobile station 50 the direction ofarrival (DoA) is first estimated by using the signal received from themobile station and then adjusting the phase offset between thetransmission antennas in such a way that a beam is generated in themeasured DoA. With two reception antennas the DoA can be estimated fromthe true phase difference in the signals received from the mobilestation by the antennas. However, due especially to imperfections inmanufacture of the components of the receive chains 26, 27, totemperature effects and to differences in cable lengths, the measuredphase difference and true phase difference differs as the receive chainsintroduce an additional phase offset into the received signals.Similarly, the measured amplitudes in baseband and true amplitudes aredifferent for both receiving and transmitting chains.

In the following discussion the calibration problems relating to thephase shift and amplitude distortion are explained. To account for theabove-mentioned errors the basestation is actively calibrated eithercontinuously or at frequent intervals. In the example of FIG. 2 thecalibration is done by injecting a calibration signal into the receivechains from a signal generator 45 and measuring at the control unit 46the delay introduced into the signal by the receive chains. This yieldsphase delays θ_(RX1) for receive (uplink) chain 26 and θ_(RX2) forreceive chain 27. A similar process is applied to the transmit chainusing signal generator 44 and phase determination unit 41 to yielddelays θ_(TX1) for transmit (downlink) chain 24 and θ_(TX2) for transmitchain 25. With this information the control unit can calculate theerrors introduced due to differences between the receive and transmitchains: Δθ_(RX) and Δθ_(TX), by:Δθ_(RX)=θ_(RX1)−θ_(RX2)andΔθ_(TX)=θ_(TX1)−θ_(TX2)

Here Δθ_(RX) gives the relationship between the true phase difference ofthe two antennas Δφ_(RX,TRUE) and the respective phase differencemeasured at the baseband Δφ_(RX,BB):Δφ_(TX,TRUE)=Δφ_(RX,BB)+Δθ_(RX.)

Typically, in beamforming systems Δφ_(RX,TRUE) is used to estimate theDoA for the respective mobile terminal. Similarly, Δθ_(TX) ties togetherthe phase difference that is imposed on the signal at basebandΔφ_(TX,BB) and the resulting true phase difference Δφ_(TX,TRUE) when thesignals leave the antennas, and is given byΔφ_(TX,TRUE)=Δφ_(TX,BB)+Δθ_(TX.)

In beamforming, the value of Δφ_(TX,TRUE) determines in which directiona beam is formed. Typically, both Δθ_(TX) and Δθ_(RX) need to beseparately measured by a calibration system in order to base downlinktransmission on the uplink measurements.

Consider next the calibration problem corresponding to the amplitudeweighting. In the receiver chain the following relationships between thetrue signal amplitudes α_(RX1,TRUE), α_(RX2,TRUE) in antennas and themeasured signal gains α_(RX1,BB), α_(RX2,BB) in baseband are given,α_(RX1,TRUE)=β_(RX1)·α_(RX1,BB)α_(RX2,TRUE)=β_(RX2)·α_(RX2,BB)

Here β_(RX1) and β_(RX2) represent the distortion caused by instrumentaldifferences in separate receiver chains. The similar equations are validalso for separate transmit chains. Thus,α_(TX1,TRUE)=β_(TX1)·α_(TX1,BB)α_(TX2,TRUE)=β_(TX2)·α_(TX2,BB)

However, in calibration of two-antenna system it is enough to controlthe relative gain between separate transmit chains. For that purpose wewrite

$\begin{matrix}{{\frac{\alpha_{{RX1},{TRUE}}}{\alpha_{{RX2},{TRUE}}} = {\beta_{RX} \cdot \frac{\alpha_{{RX1},{BB}}}{\alpha_{{RX2},{BB}}}}},} & {\beta_{RX} = \frac{\beta_{RX1}}{\beta_{RX2}}} \\{{\frac{\alpha_{{TX1},{TRUE}}}{\alpha_{{TX2},{TRUE}}} = {\beta_{TX} \cdot \frac{\alpha_{{TX1},{BB}}}{\alpha_{{TX2},{BB}}}}},} & {\beta_{TX} = \frac{\beta_{TX1}}{\beta_{TX2}}}\end{matrix}$where βhd RX and β_(TX) represent the distortions of relative amplitudesin antennas (true value) and baseband. In prior art systems both β_(RX)and β_(TX) need to be found. Typically this is done by using referencesignals which are received from the calibration transmitter and, on theother hand, sent to the calibration receiver.

The complexity of the above-mentioned calibration methods, and the needfor frequent calibration of the system makes it costly and clearly moredifficult to implement in a practical basestation which employs adaptiveantennas. Furthermore, in practice measuring the required phase offsetsand amplitude distortions is complicated by the fact that a basestationmust be able to communicate with a large number of mobile stations atthe same time, which requires that the measurement must not interferewith the normal base station operation.

An alternative system for phase adjustments is described in WO 99/09677.In that system the power or signal quality reported by each mobilestation with which the base station is in communication is monitored andused to adjust the transmission phase offset Δφ_(TX,BB) for that mobilestation only. In order to operate properly this requires frequentfeedback from the mobile station to compensate for the change offeasible phase offset. However, in the GSM system, for example, thefeedback rate is only two messages per second which results in slowconvergence of the tracking algorithm and limited ability to track thephase offset corresponding to a fast moving mobile terminal. There istherefore a need for an improved adaptive transmitter system.

A related problem arises during the initial configuration of multipleantenna units such as basestations that are capable of beamforming. Asindicated above, it is anticipated that transmit diversity will besupported in WCDMA base stations (B nodes). According to the presentlyproposed WCDMA standards, transmit diversity with two transmit antennabranches in the B node is possible. However, the performance increaseresulting from transmit diversity may be seriously impaired if thetransmit antenna branches are not well calibrated. Two principal kind ofcalibration errors may arise. Those are:

-   -   (a) the delay between signals from the separate antennas; and    -   (b) the amplitude difference between signals from separate        antennas.

Simulations have shown that even a delay of ½ chip between transmittedsignals will reduce the link performance of STTD by 0.5 dB in thepedestrian 3 channel. (In the vehicular channel the performance loss issmall). In addition, as the delay between signals increases beyond ½chip, the performance loss is increases rapidly. The effect of amplitudeerrors has not been well studied, but from theory it is evident thatimbalance between transmitted powers will reduce the performance of anytransmit diversity scheme.

At present the specifications for acceptable delay and amplitude errorsin WCDMA base station products is under discussion in the WCDMAstandardization body. A trade-off is having to be made between requiringsmall error tolerances and permitting loose error tolerances. Requiringsmall error tolerances would lead to enhanced system performance, but itis anticipated that it would be very costly for manufacturers to producebase station products that meet small error tolerances. This is becauseit is anticipated that such products would have to undergo a process ofaccurate calibration in the factory in order to balance the inherentdifferences in response characteristics between the components of thetransmitter antenna branches. On the other hand, permitting loose errortolerances would reduce system performance.

It would therefore be advantageous if it were possible to accuratelycalibrate a multi-antenna transmitter without expensive factory testing.

According to one aspect of the present invention there is provided Amethod for delay calibrating a multi-antenna transmitter having at leasttwo transmitter chains each with a respective antenna, the methodcomprising the steps of:

(a) transmitting signals to mobile stations by means of the antennas;

(b) determining feedback data indicative of the relative delay of thetransmitted signals; and

(c) adjusting the delay of one or more of the transmit chains on thebasis of the feedback data.

The feedback data could be explicitly or implicitly indicative of thedelay.

Further preferred features of the invention are set out in the dependentclaims.

The present invention will now be described by way of example withreference to the drawings.

FIG. 1 illustrates conventional and beamforming transmitter systems;

FIG. 2 shows the structure of an example beamforming basestation;

FIG. 3 shows the structure of an example beamforming basestation and amobile station capable of implementing the present invention

FIG. 4 is a flow diagram illustrating an algorithm for performing amethod for setting the value of phase offset Δθ_(o) or distortion β ofthe relative amplitude; and

FIG. 5 shows the parameter space from which the values of phase offsetΔθ_(o) and relative distortion β of the amplitude are searched.

The inventors of the present invention have observed that even ifbeamforming is employed, the absolute DoA (direction of arrival) haslittle importance for directing the transmission beam into the directionof the mobile terminal. Instead of concentrating on indirect measuressuch as DoA in beamforming, we may require that the following conditionsare satisfied:

${{\Delta\;\phi_{{TX},{TRUE}}} = {\Delta\;\phi_{{RX},{TRUE}}}},\mspace{14mu}{\frac{\alpha_{{TX1},{TRUE}}}{\alpha_{{TX2},{TRUE}}} = {\frac{\alpha_{{RX1},{TRUE}}}{\alpha_{{RX2},{TRUE}}}.}}$

Hence, only the phase difference and relative amplitude between signalsfrom separate antenna branches form the basis for transmission.Disregarding the small phase error that is caused by the frequencydifference between uplink and downlink (especially true when thedistance between the antenna elements is small), we may say that, withadequate accuracy, satisfying these conditions will lead to adaptationof downlink transmission to uplink measurements. It should be noted thatas part of this invention suitable time averaging can be applied toobtain the measured parameters and that the above condition can befulfilled on average over any appropriate period of time. It followsthat

${{{\Delta\;\phi_{{TX},{BB}}} + {\Delta\;\theta_{T\; X}}} = {{\Delta\;\phi_{{RX},{BB}}} + {\Delta\;\theta_{RX}}}},\;{{\beta_{T\; X} \cdot \frac{\alpha_{{TX1},{BB}}}{\alpha_{{TX2},{BB}}}} = {\beta_{R\; X} \cdot \frac{\alpha_{{RX1},{BB}}}{\alpha_{{RX2},{BB}}}}}$and the phase difference and the relative amplitude to be used at thebaseband can be obtained from

${{\Delta\;\phi_{{TX},{BB}}} = {{\Delta\;\phi_{{RX},{BB}}} + {\Delta\;\theta_{O}}}},\mspace{11mu}{\frac{\alpha_{{TX1},{BB}}}{\alpha_{{TX2},{BB}}} = {\beta \cdot \frac{\alpha_{{RX1},{BB}}}{\alpha_{{RX2},{BB}}}}},{{\Delta\;\theta_{O}} = {{\Delta\;\theta_{R\; X}} - {\Delta\;\theta_{T\; X}}}},{\beta = {\frac{\beta_{RX}}{\beta_{TX}}.}}$where

A second observation is that the phase offset Δθ_(o) and the distortionβ of the relative amplitude are purely instrumental quantities(intrinsic to the related transceiver pair) that change slowly in time,and importantly, are the same for all mobile stations being served bythis transceiver pair. If Δθ_(o) and β are determined and tracked byusing any mobile station or stations, they can be used to adapt thedownlink transmission for any mobile station simply by measuring thephase difference Δφ_(RX,BB) and relative gain α_(RX1,BB)/α_(RX2,BB) inuplink for that mobile station and applying the values of Δθ_(o) and βto obtain the values Δφ_(TX,BB) and α_(TX1,BB)/α_(TX2,BB) to be used forthat mobile station.

A third observation is that Δθ_(o) and β can be determined and trackedby using any mobile station being served by the transceiver pair. Forthis it is required that the mobile station is capable of transmittingto the transceiver reporting messages indicative of the strength orquality of signals received by the terminals from the transceiver. Thesemessages are used to adjust estimates of the values Δθ_(o) and β in sucha way that the true value with adequate accuracy follows.

FIG. 5 shows the parameter space and a certain parameter point (β, Δθ₀).In calibration the aim is to find a point (β,Δθ₀) such that the strengthor quality of signals received by mobiles is maximized. It is importantto note that (β,Δθ₀) maximize the strength or quality of signalsreceived by mobiles is the same for all mobiles. This two-dimensionaloptimisation problem can be solved in practice, for example, by reducingit into two consecutive one-dimensional problems. Hence, β is firstfixed and best value for Δθ₀ is searched using method proposed in FIG.4. Then Δθ₀ is fixed and best value for β is searched using method ofFIG. 4. This process is continued until feasible values for both Δθ₀ andβ are found. It is remarked that the method of FIG. 4 is applicable whenbest values for both Δθ₀ and β are searched. There exist many possiblealternatives how to determine and track Δθ₀ and β which can beimplemented within the scope of the present invention.

In FIG. 3 like components are numbered as in FIG. 2.

The basestation of FIG. 3 is a beamforming basestation. The mobilestation 60 has an antenna 61, a received signal strength or qualitymeasurement unit 62 coupled to the antenna for measuring the receivedsignal strength (RSS) or quality and reporting it to a control unit 63,and a transmission signal generation unit 64 also coupled to the antennafor generating signals for transmission under the control of the controlunit 63. The basestation has a signal strength or quality reportprocessing unit 70 which decodes the signal strength or quality reportsreceived by the base stations and processes them accordingly. Manycommunication systems require mobile stations to be capable of reportingreceived signal strength or quality to the basestation. Examples are GSM(Global System for Mobile Communications) and UMTS (Universal MobileTelecommunications System). The principles behind measurement ofreceived signal strength or quality, encoding signals strength orquality reports at mobile stations and decoding them at the base stationare well known.

The system of FIG. 3 uses the assumption that within a small period oftime differences in the transmit and receive chains will have the sameeffect for communications between the base station and all the mobilestations with which it communicates. Thus during that period Δθ_(o) canbe assumed to be the same for communications with all mobile stations.

During operation of the system of FIG. 3 a current value of Δθ_(o) isstored by control unit 70. The determination of that value is discussedbelow. When a signal from a mobile station is received the phasedifference Δφ_(RX,BB) between the signals received from that mobilestation via the two antennas is determined at control unit 80. A phasedifference Δφ_(TX,BB) is applied to signals for transmission to thatmobile station. Δφ_(TX,BB) is calculated by:Δφ_(TX,BB)=Δφ_(RX,BB)+Δθ_(o)

This expression holds for all mobile stations.

Once an initial value of Δθ_(o) has been determined, an iterativeprocess is performed to update the value, initially to improve itsaccuracy, and then to cope with temperature and other environmentalvariations. In each step of the iterative process a modification is madeto the value of Δθ_(o). The averages of the reported received signalstrengths or qualities from each of the mobile stations to which thebase station transmits before and after the modification are compared.If the average is greater after the modification then the modificationis taken to have resulted the value of Δθ_(o) more accurately reflectingthe differences introduced by the basestation hardware. In that case themodified value of Δθ_(o) is kept as a starting value for the nextiteration. Otherwise, Δθ_(o) is restored to its value beforemodification as the starting value for the next iteration. Other methodscould be used to adjust Δθ_(o).

The iterative process is illustrated in FIG. 4.

FIG. 3 shows details of the components used in the control unit 70 toperform the process. The value of Δθ_(o) is stored in store 71. Store 71is available to the transmission section 80 of the basestation forforming signals for transmission to mobile stations. A new value ofΔθ_(o) is formed in calculation unit 72. The old value of Δθ_(o) isstored in backup store 73 and the new value of Δθ_(o) is stored in store71. Signals are transmitted to the mobile stations using the value ofΔθ_(o) stored in store 71. Measurement reports from mobile stations aredetected by a signal monitor 91 in the decoding section 90 of thebasestation and passed to an averaging unit 74 which forms an average ofthe reports received over a predetermined time period. That new averageis compared by the controller 72 with the previously determined averagewhich has been stored in store 75. If the new average is greater thenthe value of Δθ_(o) stored in store 71 is left unchanged. Otherwise, thevalue of Δθ_(o) is restored to the old value of Δθ_(o) as stored inbackup store 73. The newly determined average is then loaded into store75 for use in the next iteration.

The control unit 70 also includes a set of stores 76 each of whichstores the value of Δφ_(RX,BB) for a respective mobile station. Thestores 76 are accessible to the transmission unit 80 for use in formingtransmissions to the mobile stations.

In forming a transmission to a mobile station the transmission unit 80receives a signal for transmission at 34. It applies that signal to thetransmission input 90 of the first transceiver unit 24, 26 etc. It alsoapplies the signal from phase shifter 81 to the transmission input 91 ofthe second transceiver unit 24, 26 etc. The phase shift applied by thephase shifter 81 is determined as described above using the value ofΔθ_(o) derived from store 71 and the appropriate value of Δφ_(RX,BB)derived from store 76. The appropriate value of Δφ_(RX,BB) is the valueof Δφ_(RX,BB) for the mobile station to which the signal is to bedirected. The identity of that mobile station may be determined by thetransmission unit 80 from the content of the signal itself, or from aseparate signal it receives.

Conveniently, the RSS is reported by the mobile stations according tothe normal means as required by the standard to which they operate. ThusGSM mobile stations will typically provide reports of RSS around twiceeach second, whereas UMTS mobile stations will typically provide veryfrequent reporting. If the RSS reports are very frequent then it may bepreferable to average them over time in order to remove the effect offast fades.

In order to determine the average RSS for use in refining the value ofΔθ_(o) the base station could use RSS reports from all of the mobilestations that report to the base station on the power received from thatbase station (all the mobile stations connected to that base station).Alternatively, just a subset of those mobile stations could be used inorder to make the process of determining the average RSS quicker.Reports from a single mobile station could be used if desired.

In order to determine whether the average RSS has risen or fallen as aresult of an adjustment of the value of Δθ_(o), all the reported RSSvalues could be averaged at each iterative step and the values reportedat successive steps compared with each other. Alternatively, the systemcould determine whether the majority of individual RSS values from eachmobile station have resin or fallen as a result of the adjustment. Otherschemes could also be used.

When the value of Δθ_(o) that is to be used for communications with allmobile stations is known, it is very straightforward for the basestationto begin beamforming to a mobile station that has newly attached to thebasestation. All that is needed is for the control unit 80 to measurethe difference in phase between signals received from the base stationvia the two antennas of the basestation and to use that difference asthe value of Δφ_(RX,BB) for communications with that mobile station. Ina typical basestation the phase difference can conveniently be measuredat baseband. The value of Δφ_(RX,BB) can be measured each time acommunication is received from a mobile station, or periodically. Thepreferred interval for measuring Δφ_(RX,BB) will depend on the width ofthe beam formed by the antennas, the sensitivity of the mobile stationand the expected maximum speed of the mobile station. The measured valueof Δφ_(RX,BB) may be averaged over a short timebase to give a workingvalue of Δφ_(RX,BB). The control unit 80 conveniently stores values ofΔφ_(RX,BB) to be used for communications with each mobile stationattached to the base station so that signals can be beamformed to themobile stations with little delay.

Using the same value of Δθ_(o) for communications with all mobilestations may be expected to involve some additional error over a systemin which individual values of Δθ_(o) are used for each mobile station,due to differences in frequency between the transmit and receive signalsand due to differences between the signals to and from the differentmobile stations. Since there is a spacing between the two antennas thepath lengths between a mobile station and each antenna will normally bedifferent and there will be therefore be a frequency-related componentin the phase offset as received at the antennas. However, in mostsystems the relative frequency difference between uplink and downlinksignals will be small—typically less than 10%. Therefore, thebeamforming capability of a system as described above is unlikely to behindered significantly by those errors. In addition, error can bereduced by closer spacing of the antennas; preferably the antennas areset at a spacing of ½λ, where λ is the typical wavelength at which thesystem is to operate.

When the process described above is initiated, an initial value ofΔθ_(o) must be selected. The initial value of Δθ_(o) may be preset inthe base station, determined randomly or determined by internalcalibration using signal generators 44, 45 in the base station asdiscussed above with reference to FIG. 2 and using the equationΔθ_(o)=Δθ_(RX)−Δθ_(TX).

The modification of the value of Δθ_(o) at each iteration may beperformed according to standard techniques for iterative optimisation offeedback parameters. For example, at each iteration a predeterminedsmall offset δ could be applied to the starting value of Δθ_(o) for thatiteration. δ could be added or subtracted in alternate iterations, orcould be applied with the same sign as in the previous iteration if theprevious iteration resulted in a change in the value of Δθ_(o) or withthe opposite sign if the previous iteration resulted in the value ofΔθ_(o) remaining unchanged.

The application of the above principles to the calibration of devicessuch as base stations for reduction of errors due to delay will now bedescribed.

As discussed above, due to differences between the characteristics ofthe hardware that makes up the transmit antenna branches of amulti-antenna base station, there can be differences in delay betweenthe branches. These mean that the branches impose different delay ontransmitted signals. In the case of WCDMA these differences in timedelay mean that the spreading codes of signals transmitted to differentmobile stations may no longer be truly orthogonal, resulting inintra-cell downlink interference that can reduce downlinksignal-to-noise ratio and therefore available data rate. In beamformingapplications, differences in delay may mean that the beams are notoptimally formed. One option for overcoming this is to accuratelycalibrate the base station at the factory, but this requires extensivetesting and adjustment which increases the cost of the equipment. Also,with this means of calibration alone, if calibration is later lost forsome reason the performance of the base station is degraded; and ifprojected dual-antenna features that will need still more accuratecalibration are introduced, then complicated changes may be required.

The principles described above can be used to enable open- and/orclosed-loop calibration during operation of the base station. This maybe supplemented by some initial, possible quite loose, calibration atthe factory before a base station is shipped.

Closed-loop calibration involves calibrating the base station based onreports received from mobile stations (or dedicated monitoring units)during the operation of the base station. The mobiles should be able todirectly or indirectly estimate the parameters that need to becalibrated. The results of that estimation is fed back to the basestation, where it is input to a calibration unit which, based on thatinformation, can alter the parameters of at least one of the antennabranches so as to improve the branches' calibration. In this way thecalibration can be improved during operation, although there is someincrease in signalling overhead since the mobile stations must signalthe results of their estimations to the base station. This data ispreferably signalled directly, over the radio uplink from the mobilestations to the base station.

It is preferred that there is some initial calibration at the factory,so as to ready the base station for satisfactory (even if marginal) usein the field. The accuracy of the initial calibration can, for instance,be +/−1 chip. The mobile station (or user equipment UE) measures delayand amplitude differences between pilot signals transmitted by the basestation. This is possible since separate antenna branches use orthogonalP-CPICHs. In practice, the delay between signals can be estimated ifchannel taps corresponding to separate signals are trackedsimultaneously. After computing the instant time delay and amplitudedifference, the UE can estimate the average time delay and amplitudedifference using simple FIR or IIR filters. To reduce the bandwidthrequired for reporting, the result concerning to time delay can bequantized, for example, as follows

Time −1.0 −0.5 −0.25 −0.125 +0.125 +0.25 +0.5 +1.0 Delay [chip] Coding111 011 001 101 010 110 100 000

On top of this coding, some stronger binary code can be employed. Thiscalibration information can be included into the measurement report thatis send to the base station.

At the base station, feedback information from several mobiles iscombined. We can compute, for example

1. Average feedback over all mobiles

2. Weighted average feedback over all mobiles

3. Weighted average feedback over some mobiles.

As appropriate.

In all cases time averaging (filtering) is also employed. If only someof the mobiles are selected into the calibration process, then theselection criteria can be based on the any parameter known from themeasurement report or uplink measurements. In order to guarantee thequality of calibration, other statistical measures beside the averagecan be computed. We can, for example, compute the variance from obtainedcalibration information and estimate the confidence level of thecalibration estimate. If estimate is not reliable enough, then it isrejected.

Another form of calibration, which may be used in addition to or insteadof closed-loop calibration is open-loop calibration. Again, there ispreferably at least a loose initial calibration made at the factory.Then more accurate calibration is achieved during operation based onindirect measures. By ‘indirect measures’ is meant any information thatmay already be available concerning to system performance. Thesemeasures may be, for example, the needed transmit power in base station,filtered power control commands received from mobiles or any combinationof parameters available from the measurement reports that mobilestations are to send (according to present standards in systems such asWCDMA) to base stations. This method has the advantage that it does notrequire changes to the present standards.

In the open-loop system the accuracy of the initial calibration can beagain +/−1 chip. The basic idea is that loose calibration is seen as asystem performance loss when compared to accurately calibrated system.One method of open-loop calibration is as follows:

1. Measure the system performance using available parameters andstatistical methods.

2. Change the delay and/or amplitude calibration slightly.

3. Measure again the system performance using the same parameters andstatistical methods as in stage 1.

4. Compare results and decide whether the change in calibrationparameters has improved the system performance or not. Based on theresult of this determine a new change to be applied, and return to step2.

This scheme widely applicable since many alternative detailedembodiments are possible. One could, for example, estimate the averagetransmit power per chip over all mobiles during a certain time intervaland after the change of calibration parameters the same measure isestimated. If performance is improved, then the change is assumed tohave been in the right direction and is accepted. Otherwise, the changeis reversed, or another change made in the contrary direction. Otherperformance measures beside the average transmit power can be used. Itshould be noted that it is preferable:

-   -   To keep the change of calibration parameters small. In this way        problems arising from lost performance can be minimised.    -   That the time averaging period of the performance measures        should be long enough. Time averaging may last even days.    -   That the averaging periods should be as similar as possible from        a traffic condition point of view.    -   That other statistical parameters beside the average are        computed. In this way one can confirm that estimated averages        are reliable.    -   That beside the average transmit power/chip other measures such        as parameters reported by UE can be employed.

The above systems can also work as a backup if the initial factorycalibration is not valid, or is violated for some reason.

The above systems are not limited to mobile telephony base stations.

The present invention may be applied to any adaptive transceiver systemsthat use co-polarisation antennas or that use antennas of differentpolarisation.

The present invention may be applied to systems that transmit using morethan two antennas. In such a case the phase differences caused by thetransmit and receive chains associated with one antenna and thoseassociated with each other antenna should be determined. This can stillbe done using an iterative process based on the average reported RSS.

The mobile station could be a mobile phone. The mobile station need notactually be mobile: it could be fixed in location. The mobile stationmay be termed a terminal.

The basestation and the mobile station are suitable operable accordingto any suitable protocol, for example GSM, UMTS (3G) or a derivativethereof.

The applicant draws attention to the fact that the present inventionsmay include any feature or combination of features disclosed hereineither implicitly or explicitly or any generalisation thereof, withoutlimitation to the scope of any definitions set out above. In view of theforegoing description it will be evident to a person skilled in the artthat various modifications may be made within the scope of theinventions.

1. A method for delay calibrating a multi-antenna transmitter having atleast two transmitter chains each with a respective antenna, the methodcomprising iteratively performing the steps of: (a) transmitting signalsto mobile stations by means of the antennas; (b) determining feedbackdata indicative of the relative delay of the transmitted signals; and(c) adjusting the delay of one or more of the transmit chains on thebasis of the feedback data.
 2. A method as claimed in claim 1, whereinthe signals are transmitted in the course of traffic communications withthe mobile stations.
 3. A method as claimed in claim 2, wherein thesignals are traffic signals.
 4. A method as claimed in claim 1, whereinthe said determining step comprises: measuring at one or more of themobile stations one or more characteristics of one or more of thesignals; and reporting the results of that measurement as feedback datato the multi-antenna transmitter.
 5. A method as claimed in claim 4,wherein the characteristics include one or more of signal strength andsignal quality.
 6. A method as claimed in claim 5, wherein the signalstrength is represented by signal to interference ratio.
 7. A method asclaimed in claim 5, wherein the signal quality is represented bytransport channel block error rate.
 8. A method as claimed in claim 4,wherein the step of measuring comprises measuring at more than one ofthe mobile stations one or more characteristics of one or more of thesignals; and the step of adjusting comprises averaging the feedback dataover a predetermined period to determine an averaged measurement result,and adjusting the delay of one or more of the transmit chains on thebasis of the averaged measurement result.
 9. A method as claimed inclaim 8, wherein the predetermined period is longer than one hour.
 10. Amethod as claimed in claim 8, wherein the predetermined period is longerthan 24 hours.
 11. A method as claimed in claim 1, wherein the saiddetermining step comprises: measuring at the transmitter one or morecharacteristics of one or more of the signals; and the feedback datacomprises the results of that measurement.
 12. A method as claimed inclaim 11, wherein the said characteristics measured at the transmitterinclude one or more of total transmit power and average transmit powerper chip over a predetermined period.
 13. A method as claimed in claim1, wherein the said determining step comprises: measuring at one or moreof the mobile stations one or more characteristics of one or more of thesignals; reporting the results of that measurement to the multi-antennatransmitter; measuring at the transmitter one or more characteristics ofone or more of the signals; and forming the feedback data based on bothof the said measurements.
 14. A method as claimed in claim 13, whereinthe step of forming the feedback data based on both of the saidmeasurements comprises comparing the results of the both of saidmeasurements and forming the feedback data so as to be representative ofthe relative delay as indicated by both of the said measurements.
 15. Amethod as claimed in claim 1, wherein the said transmitting, determiningand adjusting steps are performed repeatedly so as to reduce differencesin the instrumental properties of the transmitter chains in thetransceiver.
 16. A method as claimed in claim 1, comprising adjusting atleast one other characteristic of the transmitter based on the feedbackdata.
 17. A method as claimed in claim 16, wherein the othercharacteristic is the power of a signal to be transmitted to at leastone of the mobile stations.
 18. A method as claimed in claim 1,comprising adjusting the relative amplitude of signals to be transmittedby one or more of the transmit chains on the basis of the feedback data.19. A method as claimed in claim 1, comprising adjusting the relativephase of signals to be transmitted by one or more of the transmit chainson the basis of the feedback data.
 20. A method as claimed in claim 1,wherein the transmitter is a base station.
 21. A method as claimed inclaim 1, wherein the transmitter and the mobile stations are operableaccording to the 3G telecommunication protocol.
 22. A method as claimedin claim 1, wherein the transmitter is a beamforming transmitter.
 23. Amethod as claimed in claim 1, wherein the transmitter is a space-timecoding transmitter.
 24. A communication system comprising amulti-antenna transmitter having at least two transmitter chains eachwith a respective antenna, and a plurality of mobile stations, whereinthe system is arranged to perform delay calibration of the transmitterby iteratively performing the steps of: (a) transmitting signals tomobile stations by means of the antennas; (b) determining feedback dataindicative of the relative delay of the transmitted signals; and (c)adjusting the delay of one or more of the transmit chains on the basisof the feedback data.